Numerous expansion processes are commonly used for hydrocarbon liquids recovery in the gas processing industry, and particularly in the recovery of ethane and propane from high pressure feed gas. Where the feed gas pressure is relatively low or contains significant quantity of propane and heavier components, additional external (e.g., propane) refrigeration may also be required.
In most known NGL (natural gas liquids) expander processes, feed gas is cooled to a relatively low temperature to achieve partial condensation, typically by heat exchange with the demethanizer overhead vapor, side reboilers, and/or external propane refrigeration. The so condensed portion containing less volatile components is separated from the vapor portion that is typically split into two portions, with one portion being further chilled and fed to the upper section of the demethanizer while the other portion is letdown in pressure in a turbo-expander and fed to the mid section of the demethanizer. Such known configurations are commonly used for high ethane recovery for feed gas with low to medium CO2 content (less than 2%) and high C3+ content (hydrocarbon compounds with three or more carbon atoms greater than 5%), and are generally not applicable for feed gas with high CO2 content (greater than 2%) and low C3+ content (less than 2% and typically less than 1%). Among other reasons, such known processes have a significant intolerance to CO2 freezing, especially where the CO2 to C2+ (hydrocarbon compounds with two or more carbon atoms) ratio in the feed gas increases.
Moreover, in many expander processes, the residue gas from the fractionation column still contains significant amounts of ethane and propane hydrocarbons that could be further recovered if chilled to an even lower temperature, and/or subjected to another rectification stage. To that end, lower temperature can typically be achieved by a higher expansion ratio across the turbo-expander (by lowering the column pressure and temperature). However, in most known configurations, high ethane recovery in excess of 90% is neither achievable due to CO2 freezing in the demethanizer, nor economically justified due to the high capital cost of the compression equipment and energy costs. In other known NGL processes, relatively high propane recoveries can be achieved for a rich feed gas with relatively high CO2 content as very low demethanizer temperatures are not required due to the dilution effect from the presence of heavier hydrocarbons. However, such plants are then limited to a relatively low level of ethane recovery of typically 40%, or even less.
Consequently, known expander processes typically only handle feed gases with low CO2 content and rich feed gases where high propane, and especially high ethane recoveries are desirable. Where needed, a CO2 removal unit (e.g. MDEA treating) can be installed to allow feed gases with elevated CO2 content. However, such approach adds significant cost to the NGL recovery plant. Moreover, most of the known processes are also problematic when the CO2 content in the feed gas gradually increases over time, as such processes often become inoperable due to CO2 freezing in the demethanizer.
Exemplary NGL recovery plants with a turbo-expander, feed gas chiller, separators, and a refluxed demethanizer are described, for example, in U.S. Pat. No. 4,854,955 to Campell et al. Here, a configuration with turbo-expansion is employed for ethane recovery in which the demethanizer column overhead vapor is cooled and condensed by an overhead exchanger using refrigeration generated from feed gas chilling. Such additional cooling step condenses most of the propane and heavier components from the column overhead gas, which is later recovered in a separator and returned to the column as reflux. Unfortunately, while high propane recovery can be achieved with such processes, ethane recovery is often limited to less than desirable levels by CO2 freezing in the demethanizer, particularly when processing a high CO2 and lean feed gas.
Most of heretofore known plants require very low temperatures (−100° F. or lower) in the demethanizer in order to achieve a high ethane recovery. Unfortunately, due to the very low temperatures, the CO2 content in the top section of the demethanizer increases, which invariably causes significant internal recycle and accumulation of the CO2 components. Consequently, such configurations (especially when processing lean gases) are prone to CO2 freezing which presents a significant obstacle for continuous operation.
To circumvent the CO2 freezing problems in the demethanizer, several NGL recovery plants have been described that include a CO2 removal process in the NGL fractionation column. For example, U.S. Pat. No. 6,182,469 Campell et al., teaches a configuration in which a portion of the liquid in the top trays of the demethanizer is withdrawn, heated, and returned to the lower section of the column for CO2 removal and control. While this approach can reduce undesirably high CO2 concentrations to some degree, fractionation efficiency of the demethanizer is sacrificed and additional fractionation trays, heating and cooling duties must be added for the extra processing step. In yet another approach, deethanizer overhead vapor is recycled to the mid section of the demethanizer for the removal of CO2 as disclosed in U.S. Pat. No. 6,516,631 to Trebble. Such recycle scheme can also be used to reduce the CO2 content in the NGL product to some degree, but the required energy for the recycle compressor, and additional heating/cooling duties tend to render this scheme uneconomical.
Thus, numerous attempts have been made to improve the efficiency and economy of processes for separating and recovering ethane and heavier natural gas liquids from natural gas and other sources. However, all or almost all of them are complex and fail to achieve economic operation for high ethane recovery for high CO2 feed gases. Therefore, there is still a need to provide improved methods and configurations for natural gas liquids recovery.